Flux-freezing breakdown in high-conductivity magnetohydrodynamic turbulence

Abstract

The idea of 'frozen-in' magnetic field lines for ideal plasmas is useful to explain diverse astrophysical phenomena, for example the shedding of excess angular momentum from protostars by twisting of field lines frozen into the interstellar medium. Frozen-in field lines, however, preclude the rapid changes in magnetic topology observed at high conductivities, as in solar flares. Microphysical plasma processes are a proposed explanation of the observed high rates, but it is an open question whether such processes can rapidly reconnect astrophysical flux structures much greater in extent than several thousand ion gyroradii. An alternative explanation is that turbulent Richardson advection brings field lines implosively together from distances far apart to separations of the order of gyroradii. Here we report an analysis of a simulation of magnetohydrodynamic turbulence at high conductivity that exhibits Richardson dispersion. This effect of advection in rough velocity fields, which appear non-differentiable in space, leads to line motions that are completely indeterministic or 'spontaneously stochastic', as predicted in analytical studies. The turbulent breakdown of standard flux freezing at scales greater than the ion gyroradius can explain fast reconnection of very large-scale flux structures, both observed (solar flares and coronal mass ejections) and predicted (the inner heliosheath, accretion disks, γ-ray bursts and so on). For laminar plasma flows with smooth velocity fields or for low turbulence intensity, stochastic flux freezing reduces to the usual frozen-in condition.

Interpretation of the Structure Function of Rotation Measure in the Interstellar Medium

“…The Kolmogorov spectrum appears to be too steep to account for the shallow slope especially at scales larger than the spectral break, and it alone cannot serve as a satisfactory turbulent model to fit the various slopes of SFs from different observations. In fact, it has been known that the conventional flux-freezing concept breaks down in realistic MHD turbulence as the diffusion of magnetic field lines is mediated by fast magnetic reconnection, which is an intrinsic process inherent in MHD turbulence (Lazarian 2005(Lazarian , 2011Eyink et al 2013). It is more plausible that density and magnetic field fluctuations conform to distinct power spectra of turbulence.…”

Damping of Alfvén Waves by Turbulence and Its Consequences: From Cosmic-Ray Streaming to Launching Winds

“…Eventually, as we discussed, this should contribute to generating more turbulence in the reconnection region. Turbulence was shown in LV99 to change the nature of magnetic reconnection making it independent of plasma resistivity (see more in Kowal et al 2009, 2012, Eyink, Lazarian, Vishniac 2011, Eyink et al 2013, Eyink 2015, Lalescu et al 2015. Turbulence is being generated by reconnection thus inducing fast reconnection in the case when the initial state of magnetized plasmas is not turbulent (Beresnyak 2013, Oishi et al 2015.…”

Coronal Heating Topology: The Interplay of Current Sheets and Magnetic Field Lines

“…4 (insets), but field lines with initial separation within the dissipative range (ξ 0 10 dx, where dx=1/nx=1/2048 is the numerical grid step size) exhibit subsequently a Richardson-like superdiffusive stage (Beresnyak 2013;Eyink et al 2013;Servidio et al 2016) with mean square separation up to ∝ z 4 (blue continuous lines in Fig. 4, with respectively ξ 0 = 2.5, 6.5, and 10.5 dx), while for larger separations they transition to the diffusive regime.…”